This invention relates to a method for dispersing finely divided metal particles in a binder material. In a more particular manner, this invention concerns itself with a method for effecting the uniform distribution of sub-micron size electrically conducting or magnetic metal particles throughout an organic resinous binder.
The problem of providing an adequate method for the uniform dispersion of sub-micron size metal particles in a plastic binder material is well known. Because of their very small size, the metal particles tend to agglomerate or cluster together due to the mutual electrostatic forces which exist between them. The particles form clusters of critical size such that the electrostatic forces of attraction are balanced by the forces tending to separate them. In attempting to disperse these particles within a binder material, it has often been found to be very difficult to break up these clusters and keep them separated until the binder has solidified. The problem is very well known in the manufacture of paint where the paint pigments must be evenly dispersed in the binder in order to be effective. The paint manufacturing industry has solved the problem by the proper choice of binders and by a milling operation which breaks up the pigment clusters. Since the binder molecules themselves have a significant dipole moment, once the clusters are broken up in milling and become coated with the binder molecules, the mutual electrostatic forces between the particles are relieved and cluster formation is retarded.
The problem of effectively dispersing finely divided particles is also encountered in the fabrication of radiation absorbing materials. In such applications, it is most desirable that the composite materials have as low a dielectric constant as possible so that the impedance match of the material to free space is as good as possible. In addition, these materials often consist of either electrically conducting or magnetic particles dispersed throughout a binder material. The dielectric constant of the composite mixture of conducting particles and binder is a sensitive function of the particles separation and, therefore, it is critical that an adequate and effective dispersion be attained.
In the formation of radar absorbing materials, the binder component must also have a low dielectric constant. Therefore, the method employed in the dispersion of paint pigments which involves the use of a binder with a large dipole moment cannot be utilized in solving the dispersion problem encountered in the fabrication of a radar absorbing material.
In attempting to overcome the problem of providing for the uniform dispersion of finely divided metal particles in a plastic binder, it has been found that a uniform and effective dispersion of the metal particles can be accomplished by first admixing the particles with a finely divided insulating powdered material and then further mixing the admixture with a resinous, dielectric binder material which is likewise in finely divided form. The resulting dielectric constant of the composite mixture has been found to be considerably lower than that achieved by the method in which the metal particles are mechanically blended with the dielectric plastic binder.